Internalization of transthyretin. Evidence of a novel yet unidentified receptor-associated protein (RAP)-sensitive receptor

A Snapshot of the Most Recent Transthyretin Stabilizers

In recent years, several strategies have been developed for the treatment of transthyretin-related amyloidosis, whose complex clinical manifestations involve cardiomyopathy and polyneuropathy. In view of this, transthyretin stabilizers represent a major cornerstone in treatment thanks to the introduction of tafamidis into therapy and the entry of acoramidis into clinical trials. However, the clinical treatment of transthyretin-related amyloidosis still presents several challenges, urging the development of new and improved therapeutics. Bearing this in mind, in this paper, the most promising among the recently published transthyretin stabilizers were reviewed. Their activity was described to provide some insights into their clinical potential, and crystallographic data were provided to explain their modes of action. Finally, structure-activity relationship studies were performed to give some guidance to future researchers aiming to synthesize new transthyretin stabilizers. Interestingly, some new details emerged with respect to the previously known general rules that guided the design of new compounds.

Tetrameric Transthyretin as a Protective Factor Against Alzheimer's Disease

Transthyretin (TTR) is a tetrameric protein traditionally recognized for its role in transporting thyroxine and retinol. Recent research has highlighted the potential neuroprotective functions of TTR in the setting of Alzheimer's disease (AD), which is the most common form of dementia and is caused by the deposition of amyloid beta (Aβ) and the resulting cytotoxic effects. This paper explores the mechanisms of TTR protective action, including its interaction with Aβ to prevent fibril formation and promote Aβ clearance from the brain. It also synthesizes experimental evidence suggesting that enhanced TTR stability may mitigate neurodegeneration and cognitive decline in AD. Potential therapeutic strategies such as small molecule stabilizers of TTR are discussed, highlighting their role in enhancing TTR binding to Aβ and facilitating its clearance. By consolidating current knowledge and proposing directions for future research, this review aims to underscore the significance of TTR as a neuroprotective factor in AD and the potential implications for future research.

The Journey of Human Transthyretin: Synthesis, Structure Stability, and Catabolism

Transthyretin (TTR) is a homotetrameric protein mainly synthesised by the liver and the choroid plexus whose function is to carry the thyroid hormone thyroxine and the retinol-binding protein bound to retinol in plasma and cerebrospinal fluid. When the stability of the tetrameric structure is lost, it breaks down, paving the way for the aggregation of TTR monomers into insoluble fibrils leading to transthyretin (ATTR) amyloidosis, a progressive disorder mainly affecting the heart and nervous system. Several TTR gene mutations have been characterised as destabilisers of TTR structure and are associated with hereditary forms of ATTR amyloidosis. The reason why also the wild-type TTR is intrinsically amyloidogenic in some subjects is largely unknown. The aim of the review is to give an overview of the TTR biological life cycle which is largely unknown. For this purpose, the current knowledge on TTR physiological metabolism, from its synthesis to its catabolism, is described. Furthermore, a large section of the review is dedicated to examining in depth the role of mutations and physiological ligands on the stability of TTR tetramers.

Umbilical Cord and Neonatal Transthyretin and Their Relationship to Growth and Nutrition in Preterm Infants

BACKGROUND

Transthyretin (TTR), also known as prealbumin, has been suggested as an indicator of protein and nutritional status.

OBJECTIVE

The aim of this study was to examine the maternal and umbilical cord (UC) TTR in relation to intrauterine growth, and the serum TTR of preterm infants in relation to nutritional status and growth.

METHODS

After application of exclusion criteria, 49 preterm infants (mean gestational age and birth-weight 32.9±2.9 weeks and 1822±556 g) were included in the study. Transthyretin was sampled at birth and on days 14, 28, and at discharge with growth parameters and nutritional laboratory test results.

RESULTS

Mean UC and maternal TTR were positively correlated (8.5±2.4 mg/dL and 20.4±7.0 mg/dL, r=0.31, P=0.07). Umbilical cord TTR was neither an index of maturity nor of intrauterine growth. Umbilical cord TTR was higher in females (9.4±2.6 versus 7.6±1.8 mg/dL, P=0.015). Maternal TTR was lower in twin pregnancies (16.8±4.9 versus 22.5±7.3 mg/dL, P=0.007). Although TTR levels gradually increased over time in correlation with post-menstrual and chronological ages (r=0.24, P=0.011 and r=0.40, P<0.001, respectively), there was no correlation to weight gain (r=0.10, P=0.41), nutritional status, protein intake, or nutritional laboratory test results. The only significant correlations were between TTR and glucose and triglycerides levels (r=0.51, P<0.001 for both).

CONCLUSIONS

Although TTR levels increased over time, we could not demonstrate significant correlations between TTR and indices of the nutritional status in preterm infants at birth or during the neonatal course.

Transthyretin: Its function and amyloid formation

Transthyretin (TTR), which is one of the major amyloidogenic proteins in systemic amyloidosis, forms extracellular amyloid deposits in the systemic organs such as nerves, ligaments, heart, and arterioles, and causes two kinds of systemic amyloidosis, hereditary ATTR (ATTRv) amyloidosis induced by variant TTR and aging-related wild-type ATTR (ATTRwt) amyloidosis. More than 150 different mutations, most of which are amyloidogenic, have been reported in the TTR gene. Since most disease-associated mutations affect TTR tetramer dissociation rates, destabilization of TTR tetramers is widely believed to be a critical step in TTR amyloid formation. Recently, effective disease-modifying therapies such as TTR tetramer stabilizers and TTR gene silencing therapies have been developed for ATTR amyloidosis. This study reviews the clinical phenotypes of ATTR amyloidosis, TTR features, and recent progress in promising therapies for ATTR amyloidosis.

Exploring the Physiological Role of Transthyretin in Glucose Metabolism in the Liver

Transthyretin (TTR), a 55 kDa evolutionarily conserved protein, presents altered levels in several conditions, including malnutrition, inflammation, diabetes, and Alzheimer’s Disease. It has been shown that TTR is involved in several functions, such as insulin release from pancreatic β-cells, recovery of blood glucose and glucagon levels of the islets of Langerhans, food intake, and body weight. Here, the role of TTR in hepatic glucose metabolism was explored by studying the levels of glucose in mice with different TTR genetic backgrounds, namely with two copies of the TTR gene, TTR+/+; with only one copy, TTR+/−; and without TTR, TTR−/−. Results showed that TTR haploinsufficiency (TTR+/−) leads to higher glucose in both plasma and in primary hepatocyte culture media and lower expression of the influx glucose transporters, GLUT1, GLUT3, and GLUT4. Further, we showed that TTR haploinsufficiency decreases pyruvate kinase M type (PKM) levels in mice livers, by qRT-PCR, but it does not affect the hepatic production of the studied metabolites, as determined by 1H NMR. Finally, we demonstrated that TTR increases mitochondrial density in HepG2 cells and that TTR insufficiency triggers a higher degree of oxidative phosphorylation in the liver. Altogether, these results indicate that TTR contributes to the homeostasis of glucose by regulating the levels of glucose transporters and PKM enzyme and by protecting against mitochondrial oxidative stress.

Plasma Proteome Profiling of Coronary Artery Disease Patients: Downregulation of Transthyretin-An Important Event

Coronary artery disease (CAD) is a prevalent chronic inflammatory cardiac disorder. An early diagnosis is likely to help in the prevention and proper management of this disease. As the study of proteomics provides the potential markers for detection of a disease, in the present investigation, attempt has been made to identify disease-associated differential proteins involved in CAD pathogenesis. For this study, a total of 200 selected CAD patients were considered, who were recruited for percutaneous coronary intervention (PCI) treatment. The proteomic analysis was performed using two-dimensional gel electrophoresis (2-DE) and MALDI-TOF MS/MS. Samples were also subjected to Western blot analysis, enzyme-linked immunosorbent assay (ELISA), peripheral blood mononuclear cells isolation immunofluorescence (IF) analysis, analytical screening by fluorescence-activated cell sorting (FACS), and in silico analysis. The representative data were shown as mean ± SD of at least three experiments. A total of 19 proteins were identified. Among them, the most abundant five proteins (serotransferrin, talin-1, alpha-2HS glycoprotein, transthyretin (TTR), fibrinogen-α chain) were found to have altered level in CAD. Serotransferrin, talin-1, alpha-2HS glycoprotein, and transthyretin (TTR) were found to have lower level, whereas fibrinogen-α chain was found to have higher level in CAD plasma compared to healthy, confirmed by Western blot analysis. TTR, an important acute phase transport protein, was validated low level in 200 CAD patients who confirmed to undergo PCI treatment. Further, in silico and in vitro studies of TTR indicated a downexpression of CAD in plasma as compared to the plasma of healthy individuals. Lower level of plasma TTR was determined to be an important risk marker in the atherosclerotic-approved CAD patients. We suggest that the TTR lower level predicts disease severity and hence may serve as an important marker tool for CAD screening. However, further large-scale studies are required to determine the clinical significance of TTR.

Neuronal megalin mediates synaptic plasticity-a novel mechanism underlying intellectual disabilities in megalin gene pathologies

Donnai-Barrow syndrome, a genetic disorder associated to LRP2 (low-density lipoprotein receptor 2/megalin) mutations, is characterized by unexplained neurological symptoms and intellectual deficits. Megalin is a multifunctional endocytic clearance cell-surface receptor, mostly described in epithelial cells. This receptor is also expressed in the CNS, mainly in neurons, being involved in neurite outgrowth and neuroprotective mechanisms. Yet, the mechanisms involved in the regulation of megalin in the CNS are poorly understood. Using transthyretin knockout mice, a megalin ligand, we found that transthyretin positively regulates neuronal megalin levels in different CNS areas, particularly in the hippocampus. Transthyretin is even able to rescue megalin downregulation in transthyretin knockout hippocampal neuronal cultures, in a positive feedback mechanism via megalin. Importantly, transthyretin activates a regulated intracellular proteolysis mechanism of neuronal megalin, producing an intracellular domain, which is translocated to the nucleus, unveiling megalin C-terminal as a potential transcription factor, able to regulate gene expression. We unveil that neuronal megalin reduction affects physiological neuronal activity, leading to decreased neurite number, length and branching, and increasing neuronal susceptibility to a toxic insult. Finally, we unravel a new unexpected role of megalin in synaptic plasticity, by promoting the formation and maturation of dendritic spines, and contributing for the establishment of active synapses, both in in vitro and in vivo hippocampal neurons. Moreover, these structural and synaptic roles of megalin impact on learning and memory mechanisms, since megalin heterozygous mice show hippocampal-related memory and learning deficits in several behaviour tests. Altogether, we unveil a complete novel role of megalin in the physiological neuronal activity, mainly in synaptic plasticity with impact in learning and memory. Importantly, we contribute to disclose the molecular mechanisms underlying the cognitive and intellectual disabilities related to megalin gene pathologies.

Transport of the Thyroid Hormone Carrier Protein Transthyretin into Human Epidermoid Cells

Purpose: Transthyretin (TTR) is a protein with a growing number of biological functions in addition to its well-established binding and circulatory transport of thyroxine, and indirect retinoid transport through interaction with retinol-binding protein. Misfolded and aggregated wild-type and mutant TTRs are involved in amyloid diseases. Several aspects of TTR pathology and physiology remain poorly understood. Receptor-mediated cellular transport of TTR has been described in a few cell types; and such studies suggest the possibility of different TTR receptors and endocytic pathways. Our main objective was to further understand the endocytic pathways for TTR.Methods: In the current study, analyses of TTR endocytic transport were performed in the human A431 cell line. The results of TTR uptake were compared with those of the iron-carrier protein transferrin (Tf, a common stardard for endocytosis studies) in the same cell types.Results: A comparison of TTR and Tf endocytosis suggested similar early, 5-10 min, accumulation kinetics. But at a later time, 30 min, TTR accumulation was 20-30% lower than that of Tf (p < .05), a result that suggests different post-endocytic fates for these two ligands. Through the use of multiple endocytosis inhibitors, biochemical evidence is provided for an internalization pathway that differs from the clathrin-mediated endocytosis of Tf.Conclusions: These results for A431 cells are compared with others reported for different cell types; and it is suggested that this same hormone carrier protein can transit into cells through multiple endocytic pathways.

Implications of Heparan Sulfate and Heparanase in Amyloid Diseases

Amyloidosis refers to a group of diseases characterized by abnormal deposition of denatured endogenous proteins, termed amyloid, in the affected organs. Analysis of biopsy and autopsy tissues from patients revealed the presence of heparan sulfate proteoglycans (HSPGs) along with amyloid proteins in the deposits. For a long time, HSPGs were believed to occur in the deposits as an innocent bystander. Yet, the consistent presence of HSPGs in various deposits, regardless of the amyloid species, led to the hypothesis that these macromolecular glycoconjugates might play functional roles in the pathological process of amyloidosis. In vitro studies have revealed that HSPGs, or more precisely, the heparan sulfate (HS) side chains interact with amyloid peptides, thus promoting amyloid fibrillization. Although information on the mechanisms of HS participation in amyloid deposition is limited, recent studies involving a transgenic mouse model of Alzheimer's disease point to an active role of HS in amyloid formation. Heparanase cleavage alters the molecular structure of HS, and thus modulates the functional roles of HS in homeostasis, as well as in diseases, including amyloidosis. The heparanase transgenic mice have provided models for unveiling the effects of heparanase, through cleavage of HS, in various amyloidosis conditions.

Scavenger Receptor Class B Member 1 Independent Uptake of Transthyretin by Cultured Hepatocytes Is Regulated by High Density Lipoprotein

Thyroid hormone (thyroxine, T4) is essential for the normal function of all cell types and is carried in serum bound to several proteins including transthyretin. Recently, evidence has emerged of alternate pathways for hormone entry into cells that are dependent on hormone binding proteins. Transthyretin and transthyretin bound T4 are endocytosed by placental trophoblasts through the high-density lipoprotein receptor, Scavenger Receptor Class B Type 1 (SR-B1). High density lipoprotein (HDL) affects the expression and function of SR-B1 in trophoblast cells. SR-B1 is also expressed in hepatocytes and we sought to determine if hepatocyte SR-B1 was involved in transthyretin or transthyretin-T4 uptake and whether uptake was affected by HDL. Transthyretin and transthyretin-T4 uptake by hepatocytes is not dependent on SR-B1. HDL treatment reduced SR-B1 expression. However, pretreatment of hepatocytes with HDL increased uptake of transthyretin-T4. Knockdown of SR-B1 expression using siRNA also increased transthyretin-T4 uptake. Coaddition of HDL to transthyretin uptake experiments blocked both transthyretin and transthyretin-T4 uptake. Hepatocyte uptake of transthyretin-T4 uptake is influenced by, but is not dependent on, SR-B1 expression. HDL also decreases transthyretin-T4 uptake and therefore diet or drugs may interfere with this process. This suggests that multiple lipoprotein receptors may be involved in the regulation of uptake of transthyretin-T4 in a cell-type specific manner. Further study is required to understand this important process.

Transthyretin uptake in placental cells is regulated by the high-density lipoprotein receptor, scavenger receptor class B member 1

Transfer of thyroid hormone into cells is critical for normal physiology and transplacental transfer of maternal thyroid hormones is essential for normal fetal growth and development. Free thyroid hormone is known to enter cells through specific cell surface transport proteins, and for many years this uptake of unbound thyroid hormones was assumed to be the only relevant mechanism. Recently, evidence has emerged of alternate pathways for hormone entry into cells that are dependent on hormone binding proteins. In this study we identify the high-density lipoprotein receptor Scavenger Receptor class B member 1 (SR-B1) as important in the uptake and transport of transthyretin-bound thyroid hormone by placental trophoblast cells. High-density lipoprotein increases expression of SR-B1 in placental cells but also reduces uptake of transthyretin-thyroid hormone through the SR-B1 transporter. SR-B1 is expressed in many cells and this study suggests that SR-B1 may be universally important in thyroid hormone uptake. Further investigation of SR-B1-TTR interactions may fundamentally change our understanding of hormone biology and have important clinical consequences.

Cellular clearance of circulating transthyretin decreases cell-nonautonomous proteotoxicity in Caenorhabditis elegans

Cell-autonomous and cell-nonautonomous mechanisms of neurodegeneration appear to occur in the proteinopathies, including Alzheimer's and Parkinson's diseases. However, how neuronal toxicity is generated from misfolding-prone proteins secreted by nonneuronal tissues and whether modulating protein aggregate levels at distal locales affects the degeneration of postmitotic neurons remains unknown. We generated and characterized animal models of the transthyretin (TTR) amyloidoses that faithfully recapitulate cell-nonautonomous neuronal proteotoxicity by expressing human TTR in the Caenorhabditis elegans muscle. We identified sensory neurons with affected morphological and behavioral nociception-sensing impairments. Nonnative TTR oligomer load and neurotoxicity increased following inhibition of TTR degradation in distal macrophage-like nonaffected cells. Moreover, reducing TTR levels by RNAi or by kinetically stabilizing natively folded TTR pharmacologically decreased TTR aggregate load and attenuated neuronal dysfunction. These findings reveal a critical role for in trans modulation of aggregation-prone degradation that directly affects postmitotic tissue degeneration observed in the proteinopathies.

Neuron-derived transthyretin modulates astrocytic glycolysis in hormone-independent manner

It has been shown that neurons alter the expression of astrocytic metabolic enzymes by secretion of until now unknown molecule(s) into extracellular fluid. Here, we present evidence that neuron-derived transthyretin (TTR) stimulates expression of glycolytic enzymes in astrocytes which is reflected by an increased synthesis of ATP. The action of TTR is restricted to regulatory enzymes of glycolysis: phosphofructokinase P (PFKP) and pyruvate kinase M1/M2 isoforms (PKM1/2). The regulation of PFK and PKM expression by TTR is presumably specific for brain tissue and is independent of the role of TTR as a carrier protein for thyroxine and retinol. TTR induced expression of PKM and PFK is mediated by the cAMP/PKA-dependent pathway and is antagonized by the PI3K/Akt pathway. Our results provide the first experimental evidence for action of TTR as a neuron-derived energy metabolism activator in astrocytes and describe the mechanisms of its action. The data presented here suggest that TTR is involved in a mechanism in which neurons stimulate degradation of glycogen-derived glucosyl units without significant modulation of glucose uptake by glial cells.

Transthyretin provides trophic support via megalin by promoting neurite outgrowth and neuroprotection in cerebral ischemia

Transthyretin (TTR) is a protein whose function has been associated to binding and distribution of thyroid hormones in the body and brain. However, little is known regarding the downstream signaling pathways triggered by wild-type TTR in the CNS either in neuroprotection of cerebral ischemia or in physiological conditions. In this study, we investigated how TTR affects hippocampal neurons in physiologic/pathologic conditions. Recombinant TTR significantly boosted neurite outgrowth in mice hippocampal neurons, both in number and length, independently of its ligands. This TTR neuritogenic activity is mediated by the megalin receptor and is lost in megalin-deficient neurons. We also found that TTR activates the mitogen-activated protein kinase (MAPK) pathways (ERK1/2) and Akt through Src, leading to the phosphorylation of transcription factor CREB. In addition, TTR promoted a transient rise in intracellular calcium through NMDA receptors, in a Src/megalin-dependent manner. Moreover, under excitotoxic conditions, TTR stimulation rescued cell death and neurite loss in TTR KO hippocampal neurons, which are more sensitive to excitotoxic degeneration than WT neurons, in a megalin-dependent manner. CREB was also activated by TTR under excitotoxic conditions, contributing to changes in the balance between Bcl2 protein family members, toward anti-apoptotic proteins (Bcl2/BclXL versus Bax). Finally, we clarify that TTR KO mice subjected to pMCAO have larger infarcts than WT mice, because of TTR and megalin neuronal downregulation. Our results indicate that TTR might be regarded as a neurotrophic factor, because it stimulates neurite outgrowth under physiological conditions, and promotes neuroprotection in ischemic conditions.

Prions and Protein Assemblies that Convey Biological Information in Health and Disease

Prions derived from the prion protein (PrP) were first characterized as infectious agents that transmit pathology between individuals. However, the majority of cases of neurodegeneration caused by PrP prions occur sporadically. Proteins that self-assemble as cross-beta sheet amyloids are a defining pathological feature of infectious prion disorders and all major age-associated neurodegenerative diseases. In fact, multiple non-infectious proteins exhibit properties of template-driven self-assembly that are strikingly similar to PrP. Evidence suggests that like PrP, many proteins form aggregates that propagate between cells and convert cognate monomer into ordered assemblies. We now recognize that numerous proteins assemble into macromolecular complexes as part of normal physiology, some of which are self-amplifying. This review highlights similarities among infectious and non-infectious neurodegenerative diseases associated with prions, emphasizing the normal and pathogenic roles of higher-order protein assemblies. We propose that studies of the structural and cellular biology of pathological versus physiological aggregates will be mutually informative.

Curcumin: A multi-target disease-modifying agent for late-stage transthyretin amyloidosis

Transthyretin amyloidoses encompass a variety of acquired and hereditary diseases triggered by systemic extracellular accumulation of toxic transthyretin aggregates and fibrils, particularly in the peripheral nervous system. Since transthyretin amyloidoses are typically complex progressive disorders, therapeutic approaches aiming multiple molecular targets simultaneously, might improve therapy efficacy and treatment outcome. In this study, we evaluate the protective effect of physiologically achievable doses of curcumin on the cytotoxicity induced by transthyretin oligomers in vitro by showing reduction of caspase-3 activity and the levels of endoplasmic reticulum-resident chaperone binding immunoglobulin protein. When given to an aged Familial Amyloidotic Polyneuropathy mouse model, curcumin not only reduced transthyretin aggregates deposition and toxicity in both gastrointestinal tract and dorsal root ganglia but also remodeled congophilic amyloid material in tissues. In addition, curcumin enhanced internalization, intracellular transport and degradation of transthyretin oligomers by primary macrophages from aged Familial Amyloidotic Polyneuropathy transgenic mice, suggesting an impaired activation of naïve phagocytic cells exposed to transthyretin toxic intermediate species. Overall, our results clearly support curcumin or optimized derivatives as promising multi-target disease-modifying agent for late-stage transthyretin amyloidosis.

Transthyretin participates in beta-amyloid transport from the brain to the liver--involvement of the low-density lipoprotein receptor-related protein 1?

Transthyretin (TTR) binds Aβ peptide, preventing its deposition and toxicity. TTR is decreased in Alzheimer’s disease (AD) patients. Additionally, AD transgenic mice with only one copy of the TTR gene show increased brain and plasma Aβ levels when compared to AD mice with both copies of the gene, suggesting TTR involvement in brain Aβ efflux and/or peripheral clearance. Here we showed that TTR promotes Aβ internalization and efflux in a human cerebral microvascular endothelial cell line, hCMEC/D3. TTR also stimulated brain-to-blood but not blood-to-brain Aβ permeability in hCMEC/D3, suggesting that TTR interacts directly with Aβ at the blood-brain-barrier. We also observed that TTR crosses the monolayer of cells only in the brain-to-blood direction, as confirmed by in vivo studies, suggesting that TTR can transport Aβ from, but not into the brain. Furthermore, TTR increased Aβ internalization by SAHep cells and by primary hepatocytes from TTR+/+ mice when compared to TTR−/− animals. We propose that TTR-mediated Aβ clearance is through LRP1, as lower receptor expression was found in brains and livers of TTR−/− mice and in cells incubated without TTR. Our results suggest that TTR acts as a carrier of Aβ at the blood-brain-barrier and liver, using LRP1.

Amyloidogenic and non-amyloidogenic transthyretin variants interact differently with human cardiomyocytes: insights into early events of non-fibrillar tissue damage

TTR (transthyretin) amyloidoses are diseases characterized by the aggregation and extracellular deposition of the normally soluble plasma protein TTR. Ex vivo and tissue culture studies suggest that tissue damage precedes TTR fibril deposition, indicating that early events in the amyloidogenic cascade have an impact on disease development. We used a human cardiomyocyte tissue culture model system to define these events. We previously described that the amyloidogenic V122I TTR variant is cytotoxic to human cardiac cells, whereas the naturally occurring, stable and non-amyloidogenic T119M TTR variant is not. We show that most of the V122I TTR interacting with the cells is extracellular and this interaction is mediated by a membrane protein(s). In contrast, most of the non-amyloidogenic T119M TTR associated with the cells is intracellular where it undergoes lysosomal degradation. The TTR internalization process is highly dependent on membrane cholesterol content. Using a fluorescent labelled V122I TTR variant that has the same aggregation and cytotoxic potential as the native V122I TTR, we determined that its association with human cardiomyocytes is saturable with a KD near 650 nM. Only amyloidogenic V122I TTR compete with fluorescent V122I for cell-binding sites. Finally, incubation of the human cardiomyocytes with V122I TTR but not with T119M TTR, generates superoxide species and activates caspase 3/7. In summary, our results show that the interaction of the amyloidogenic V122I TTR is distinct from that of a non-amyloidogenic TTR variant and is characterized by its retention at the cell membrane, where it initiates the cytotoxic cascade.

Glial cells in familial amyloidotic polyneuropathy

INTRODUCTION

Transthyretin V30M mutation is the most common variant leading to Familial Amyloidotic Polyneuropathy. In this genetic disorder, Transthyretin accumulates preferentially in the extracellular matrix of peripheral and autonomic nervous systems leading to cell death and dysfunction. Thus, knowledge regarding important biological systems for Transthyretin clearance might unravel novel insights into Familial Amyloidotic Polyneuropathy pathophysiology. Herein, our aim was to evaluate the ability of glial cells from peripheral and autonomic nervous systems in Transthyretin uptake and degradation. We assessed the role of glial cells in Familial Amyloidotic Polyneuropathy pathogenesis with real-time polymerase chain reaction, immunohistochemistry, interference RNA and confocal microscopy.

RESULTS

Histological examination revealed that Schwann cells and satellite cells, from an Familial Amyloidotic Polyneuropathy mouse model, internalize and degrade non-fibrillar Transthyretin. Immunohistochemical studies of human nerve biopsies from V30M patients and disease controls showed intracellular Transthyretin immunoreactivity in Schwann cells, corroborating animal data. Additionally, we found Transthyretin expression in colon of this Familial Amyloidotic Polyneuropathy mouse model, probably being synthesized by satellite cells of the myenteric plexus.

CONCLUSIONS

Glial cells from the peripheral and autonomic nervous systems are able to internalize Transthyretin. Overall, these findings bring to light the closest relationship between Transthyretin burden and clearance from the nervous system extracellular milieu.

Fibroblasts endocytose and degrade transthyretin aggregates in transthyretin-related amyloidosis

Transthyretin (TTR)-related amyloidosis is a fatal disorder characterized by systemic extracellular deposition of TTR amyloid fibrils. Mutations in the TTR gene cause an autosomal dominant form of the disease-familial amyloidotic polyneuropathy (FAP). Wild-type (WT) TTR can also form amyloid fibrils in elderly patients with senile systemic amyloidosis. Regression of amyloid deposits in FAP patients who undergo liver transplantation to remove the main source of mutant TTR suggests the existence of mechanisms for the clearance of TTR deposits from the extracellular matrix (ECM), but the precise mechanisms are largely unknown. Because fibroblasts are abundant, playing a central role in the maintenance of the ECM and because the skin is one of the major sites of soluble TTR catabolism, in the present study, we analyzed their role in clearance of TTR aggregates. In vitro studies with a fibroblast cell line revealed that fibroblasts endocytosed and degraded aggregated TTR. Subcutaneous injection of soluble and aggregated TTR into WT mice showed internalization and clearance over time by both fibroblasts and macrophages. Immunohistochemical studies of skin biopsies from V30M patients, asymptomatic carriers, recipients of domino FAP livers as well as transgenic mice for human V30M showed intracellular TTR immunoreactivity in fibroblasts and macrophages that increased with clinical status and with age in transgenic mice. Overall, the present in vitro and in vivo data show that fibroblasts endocytose and degrade TTR aggregates. The function or dysfunction of TTR clearance by fibroblasts may have important implications for the development, progression, and regression of TTR deposition in the ECM.

Presence of N-glycosylated transthyretin in plasma of V30M carriers in familial amyloidotic polyneuropathy: an escape from ERAD

Familial amyloid polyneuropathy (FAP) is an autosomal dominant disease characterized by deposition of amyloid related to the presence of mutations in the transthyretin (TTR) gene. TTR is mainly synthesized in liver, choroid plexuses of brain and pancreas and secreted to plasma and cerebrospinal fluid (CSF). Although it possesses a sequon for N-glycosylation N-D-S at position 98, it is not secreted as a glycoprotein. The most common FAP-associated mutation is TTR V30M. In a screening for monoclonal antibodies developed against an amyloidogenic TTR form, we detected a distinct TTR with slower electrophoretic mobility in Western of plasma from carriers of the V30M mutation, not present in normal plasma. Mass spectrometry analyses of this slower migrating TTR (SMT) identified both wild-type and mutant V30M; SMT was undetectable upon N-glycosidase F treatment. Furthermore, SMT readily disappeared in the plasma of V30M - FAP patients after liver transplantation and appeared in plasma of transplanted domino individuals that received a V30M liver. SMT was also detected in plasma, but not in CSF of transgenic mice for the human V30M mutation. A hepatoma cell line transduced to express human V30M did not present the SMT modification in secretion media. Glycosylated TTR was absent in fibrils extracted from human kidney V30M autopsy tissue or in TTR aggregates extracted from the intestine of human TTR transgenic mice. Studies on the metabolism of this novel, glycosylated TTR secreted from FAP liver are warranted to provide new mechanisms in protein quality control and etiopathogenesis of the disease.

Transthyretin binds to glucose-regulated proteins and is subjected to endocytosis by the pancreatic β-cell

Transthyretin (TTR) is a functional protein in the pancreatic β-cell. It promotes insulin release and protects against β-cell death. We now demonstrate by ligand blotting, adsorption to specific magnetic beads, and surface plasmon resonance that TTR binds to glucose-regulated proteins (Grps)78, 94, and 170, which are members of the endoplasmic reticulum chaperone family, but Grps78 and 94 have also been found at the plasma membrane. The effect of TTR on changes in cytoplasmic free Ca(2+) concentration ([Ca(2+)](i)) was abolished if the cells were treated with either dynasore, a specific inhibitor of dynamin GTPase that blocks clathrin-mediated endocytosis, or an antibody against Grp78, that prevents TTR from binding to Grp78. The conclusion is that TTR binds to Grp78 at the plasma membrane, is internalized into the β-cell via a clathrin-dependent pathway, and that this internalization is necessary for the effects of TTR on β-cell function.

Transthyretin amyloidosis and the kidney

The amyloidoses are protein-misfolding disorders associated with progressive organ dysfunction. Immunoglobulin light chain is the most common, amyloid A the longest recognized, and transthyretin-associated amyloidosis (ATTR) the most frequent inherited systemic form. Although ATTR, an autosomal-dominant disease, is associated with at least 100 different transthyretin (TTR) mutations, the single amino-acid substitution of methionine for valine at position 30 is the most common mutation. Each variant has a different organ involvement, although clinical differences attributed to environmental and genetic factors exist within the same mutation. Peripheral neuropathy and cardiomyopathy are broadly described, and insights into disease reveal that kidney impairment and proteinuria are also clinical features. This review combines clinical and laboratory findings of renal involvement from the main geographic regions of disease occurrence and for different mutations of TTR. Fifteen nephropathic variants have been described, but the TTR V30M mutation is the best documented. Nephropathy affects patients with late-onset neuropathy, low penetrance in the family, and cardiac dysrhythmias. Microalbuminuria can be the disorder's first presentation, even before the onset of neuropathy. Amyloid renal deposits commonly occur, even in the absence of urinary abnormalities. The experience with renal replacement therapy is based on hemodialysis, which is associated with poor survival. Because TTR is synthesized mainly in the liver, liver transplantation has been considered an acceptable treatment; simultaneous liver-kidney transplantation is recommended to avoid recurrence of nephropathy. In addition, the kidney-safety profile of new drugs in development may soon be available.

Transthyretin: roles in the nervous system beyond thyroxine and retinol transport

Transthyretin (TTR) is a plasma- and cerebrospinal fluid-circulating protein. Besides the primordially attributed systemic role as a transporter molecule of thyroxine (T₄) and retinol (through the binding to retinol-binding protein [RBP]), TTR has been recognized as a protein with important functions in several aspects of the nervous system physiology. TTR has been shown to play an important role in behavior, cognition, amidated neuropeptide processing and nerve regeneration. Furthermore, it has been proposed that TTR is neuroprotective in Alzheimer's disease and cerebral ischemia. Mutations in TTR are a well-known cause of familial amyloidotic polyneuropathy, an autosomal dominant neurodegenerative disorder characterized by systemic deposition of TTR amyloid fibrils, particularly in the peripheral nervous system. The purpose of this review is to highlight the roles of TTR in the nervous system, beyond its systemic role as a transporter molecule of T₄ and RBP-retinol.

CSF transthyretin neuroprotection in a mouse model of brain ischemia

Brain injury caused by ischemia is a major cause of human mortality and physical/cognitive disability worldwide. Experimentally, brain ischemia can be induced surgically by permanent middle cerebral artery occlusion. Using this model, we studied the influence of transthyretin in ischemic stroke. Transthyretin (TTR) is normally responsible for the transport of thyroid hormones and retinol in the blood and CSF. We found that TTR null mice (TTR(-/-) ) did not show significant differences in cortical infarction 24 h after permanent middle cerebral artery occlusion compared with TTR(+/+) control littermates. However, TTR null mice, heterozygous for the heat-shock transcription factor 1 (TTR(-/-) HSF1(+/-) mice), which compromised the stress response, showed a significant increase in cortical infarction, cerebral edema and the microglial-leukocyte response compared with TTR(+/+) HSF1(+/-) mice. Unexpectedly, we observed novel TTR distribution throughout the infarct, localized to disintegrated β-tubulin III(+) neurons and cell debris. Specific elimination of TTR synthesis in the liver by RNAi had no effect on TTR distribution in the infarct, indicating that the observed TTR infiltration derived from CSF and not from the serum. This finding is corroborated by results from 'in situ' hybridization and real time PCR that excluded the presence of transthyretin mRNA in the infarct and peri-infarct areas. Our data suggest that in conditions of a compromised heat-shock response, CSF TTR contributes to control neuronal cell death, edema and inflammation, thereby influencing the survival of endangered neurons in cerebral ischemia.

Aboard transthyretin: From transport to cleavage

Transthyretin (TTR) is a plasma and cerebrospinal fluid protein mainly recognized as the transporter of thyroxine (T(4)) and retinol. Mutated TTR leads to familial amyloid polyneuropathy, a neurodegenerative disorder characterized by TTR amyloid deposition particularly in peripheral nerves. Beside its transport activities, TTR is a cryptic protease and participates in the biology of the nervous system. Several studies have been directed at finding new ligands of TTR to further explore the biology of the protein. From the identified ligands, some were in fact TTR protease substrates. In this review, we will discuss the existent information concerning TTR ligands/substrates.

Transthyretin in peripheral nerve regeneration

Transthyretin (TTR) is the protein transporter of thyroxine and retinol. Several TTR mutations are associated with familial amyloid polyneuropathy, a neurodegenerative disorder characterized by extracellular deposition of TTR aggregates and fibrils in the peripheral nervous system. Several reports suggest new TTR functions in the nervous system particularly in nerve regeneration and in neuroprotection in Alzheimer’s disease. The fact that TTR increases axonal growth during peripheral nervous system, regeneration and allows an appropriate retrograde transport may represent the missing link explaining the preferential deposition of mutated TTR in the peripheral nervous system of familial amyloid polyneuropathy patients. This paper discusses the details explaining the role of TTR during nerve regeneration.

Carrier-mediated thyroid hormone transport into placenta by placental transthyretin

CONTEXT

The serum protein transthyretin (TTR) plays an important role in the transport of thyroid hormone and retinol, which are critical for normal development of the human fetus. TTR is not only synthesized and secreted into the circulation by the liver and other tissues but is also synthesized by placental trophoblasts, which separate the maternal and fetal circulations. Whether it is secreted or taken up by these cells and whether it carries thyroid hormone is unknown.

OBJECTIVE AND METHODS

Our objective was to study placental handling of TTR and determine whether TTR participates in placental thyroid hormone transport. We investigated the capacity of human placenta and choriocarcinoma cell lines to secrete and internalize TTR and its ligands by Western blotting, immunofluorescence, and uptake of radiolabeled TTR.

RESULTS

Human placental explants and TTR expressing JEG-3 cells secrete TTR. JEG-3 cells grown in bicameral chambers secrete TTR, predominantly from the apical surface. Human placental explants and JEG-3 cells internalize Alexa Fluor488-labeled TTR and (125)I-TTR. Furthermore, binding to thyroid hormones (T(4), T(3)) increases (125)I-TTR uptake by enhancing tetramer formation. Cross-linking experiments confirm internalization of the TTR-(125)I-T(4) complex.

CONCLUSIONS

Our results suggest that human placenta and choriocarcinoma cells secrete transthyretin, which binds extracellular T(4), and that T(4) binding results in increased internalization of TTR-T(4) complex. TTR production by trophoblasts may represent a mechanism to allow transfer of maternal thyroid hormone to the fetal circulation that could have important implications for fetal development.

Substrate specificity of transthyretin: identification of natural substrates in the nervous system

Besides functioning as the plasma transporter of retinol and thyroxine, TTR (transthyretin) is a protease, cleaving apoA-I (apolipoprotein A-I) after a phenylalanine residue. In the present study, we further investigated TTR substrate specificity. By using both P-diverse libraries and a library of phosphonate inhibitors, a TTR preference for a lysine residue in P1 was determined, suggesting that TTR might have a dual specificity and that, in addition to apoA-I, other TTR substrates might exist. Previous studies revealed that TTR is involved in the homoeostasis of the nervous system, as it participates in neuropeptide maturation and enhances nerve regeneration. We investigated whether TTR proteolytic activity is involved in these functions. Both wild-type TTR and TTR(prot-) (proteolytically inactive TTR) had a similar effect in the expression of peptidylglycine alpha-amidating mono-oxygenase, the rate-limiting enzyme in neuropeptide amidation, excluding the involvement of TTR proteolytic activity in neuropeptide maturation. However, TTR was able to cleave amidated NPY (neuropeptide Y), probably contributing to the increased NPY levels reported in TTR-knockout mice. To assess the involvement of TTR proteolytic activity in axonal regeneration, neurite outgrowth of cells cultivated with wild-type TTR or TTR(prot-), was measured. Cells grown with TTR(prot-) displayed decreased neurite length, thereby suggesting that TTR proteolytic activity is important for its function as a regeneration enhancer. By showing that TTR is able to cleave NPY and that its proteolytic activity affects axonal growth, the present study shows that TTR has natural substrates in the nervous system, establishing further its relevance in neurobiology.

Transthyretin internalization by sensory neurons is megalin mediated and necessary for its neuritogenic activity

Mutated transthyretin (TTR) causes familial amyloid polyneuropathy, a neurodegenerative disorder characterized by TTR deposition in the peripheral nervous system (PNS). The origin/reason for TTR deposition in the nerve is unknown. Here we demonstrate that both endogenous mouse TTR and TTR injected intravenously have access to the mouse sciatic nerve. We previously determined that in the absence of TTR, both neurite outgrowth in vitro and nerve regeneration in vivo were impaired. Reinforcing this finding, we now show that local TTR delivery to the crushed sciatic nerve rescues the regeneration phenotype of TTR knock-out (KO) mice. As the absence of TTR was unrelated to neuronal survival, we further evaluated the Schwann cell and inflammatory response to injury, as well as axonal retrograde transport, in the presence/absence of TTR. Only retrograde transport was impaired in TTR KO mice which, in addition to the neurite outgrowth impairment, might account for the decreased regeneration in this strain. Moreover, we show that in vitro, in dorsal root ganglia neurons, clathrin-dependent megalin-mediated TTR internalization is needed for TTR neuritogenic activity. Supporting this observation, we demonstrate that in vivo, decreased levels of megalin lead to decreased nerve regeneration and that megalin's action as a regeneration enhancer is dependent on TTR. In conclusion, our work unravels the mechanism of TTR action during nerve regeneration. Additionally, TTR presence in the nerve, as is here shown, may underlie its preferential deposition in the PNS of familial amyloid polyneuropathy patients.

Insulin uptake across the luminal membrane of the rat proximal tubule in vivo and in vitro

We visualized insulin uptake in vivo across the apical membrane of the rat proximal tubule (PT) by confocal microscopy; we compared it with in vitro findings in a rat PT cell line (WKPT) using fluorescence microscopy and flow cytometry. Surface tubules were observed in vivo with a 633-nm single laser-illuminated real-time video-rate confocal scanning microscope in upright configuration for optical sectioning below the renal capsule. Fields were selected containing proximal and distal tubules; Cy5-labeled insulin was injected twice (the second time after ∼140 min) into the right jugular vein, and the fluorescence signal (at 650–670 nm) was recorded. Fluorescence was detected almost immediately at the brush-border membrane (BBM) of PT cells only, moving inside cells within 30–40 min. As a measure of insulin uptake, the ratio of the fluorescence signal after the second injection to the first doubled (ratio: 2.11 ± 0.26, mean ± SE, n = 10), indicating a “priming,” or stimulating, effect of insulin on its uptake mechanism at the BBM. This effect did not occur after pretreatment with intravenous lysine (ratio: 1.03 ± 0.07, n = 6; P < 0.01). Cy2- or Cy3-labeled insulin uptake in a PT cell line in vitro was monitored by 488-nm excitation fluorescence microscopy using an inverted microscope. Insulin localized toward the apical membrane of these cells. Semiquantitative analysis of insulin uptake by flow cytometry also demonstrated a priming effect (upregulation) on insulin internalization in the presence of increasing amounts of insulin, as was observed in vivo; moreover, this effect was not seen with, or affected by, the similarly endocytosed ligand β2-glycoprotein.

Retinoids modulate expression of the endocytic partners megalin, cubilin, and disabled-2 and uptake of vitamin D-binding protein in human mammary cells

The major circulating form of vitamin D, 25-hydroxycholecalciferol (25D3), circulates bound to vitamin D-binding protein (DBP). Prior to activation to 1,25-dihydroxycholecalciferol in the kidney, the 25D3-DBP complex is internalized via receptor-mediated endocytosis, which is absolutely dependent on the membrane receptors megalin and cubilin and the adaptor protein disabled-2 (Dab2). We recently reported that mammary epithelial cells (T-47D) expressing megalin, cubilin, and Dab2 rapidly internalize DBP via endocytosis, whereas cells that do not express all 3 proteins (MCF-7) do not. The objectives of this study were to characterize megalin, cubilin, and Dab2 expression and transport of DBP in human mammary epithelial cells. Using immunoblotting and real-time PCR, we found that megalin, cubilin, and Dab2 were expressed and dose dependently induced by all-trans-retinoic acid (RA) in T-47D human breast cancer cells and that RA-treated T-47D cells exhibited enhanced DBP internalization. These are the first studies to our knowledge to demonstrate that mammary epithelial cells express megalin, cubilin, and Dab2, which are enhanced during differentiation and may explain, at least in part, our finding that receptor-mediated endocytosis of DBP is upregulated in differentiated mammary epithelial cells.

Interaction of diethylstilbestrol and ioxynil with transthyretin in chicken serum

The association of suspected endocrine-disrupting chemicals (EDCs), diethylstilbestrol (DES), ioxynil and pentachlorophenol (PCP), with chicken serum proteins was investigated in relation to thyroid system disruption. All of these chemicals strongly inhibited l-[(125)I]thyroxine ([(125)I]T(4)) binding to purified transthyretin (TTR) whereas PCP was less potent inhibitor than DES and ioxynil of [(125)I]T(4) binding to diluted whole chicken serum. This result suggested that PCP interacted with serum proteins other than TTR in whole chicken serum. Following the incubation of chicken serum with each chemical (final concentrations 0.25-1.0 microM), serum proteins were fractionated by gel filtration chromatography (Cellulofine GCL-1000) and affinity chromatography (human retinol-binding protein coupled to Sepharose 4B). Although all chemicals were detected in the gel filtration chromatography 50-100 kDa fractions, DES and ioxynil, but not PCP, were co-eluted with TTR during affinity chromatography. Our results indicated that a significant proportion of DES and ioxynil, but a low proportion of PCP, interacted with TTR in whole chicken serum.

ApoA-I cleaved by transthyretin has reduced ability to promote cholesterol efflux and increased amyloidogenicity

A fraction of plasma transthyretin (TTR) circulates in HDL through binding to apolipoprotein A-I (apoA-I). Moreover, TTR is able to cleave the C terminus of lipid-free apoA-I. In this study, we addressed the relevance of apoA-I cleavage by TTR in lipoprotein metabolism and in the formation of apoA-I amyloid fibrils. We determined that TTR may also cleave lipidated apoA-I, with cleavage being more effective in the lipid-poor prebeta-HDL subpopulation. Upon TTR cleavage, discoidal HDL particles displayed a reduced capacity to promote cholesterol efflux from cholesterol-loaded THP-1 macrophages. In similar assays, TTR-containing HDL from mice expressing human TTR in a TTR knockout background had a decreased ability to perform reverse cholesterol transport compared with similar particles from TTR knockout mice, reinforcing the notion that cleavage by TTR reduces the ability of apoA-I to promote cholesterol efflux. As amyloid deposits composed of N-terminal apoA-I fragments are common in the atherosclerotic intima, we assessed the impact of TTR cleavage on apoA-I aggregation and fibrillar growth. We determined that TTR-cleaved apoA-I has a high propensity to form aggregated particles and that it formed fibrils faster than full-length apoA-I, as assessed by electron microscopy. Our results show that apoA-I cleavage by TTR may affect HDL biology and the development of atherosclerosis by reducing cholesterol efflux and increasing the apoA-I amyloidogenic potential.

Thyroid system-disrupting chemicals: interference with thyroid hormone binding to plasma proteins and the cellular thyroid hormone signaling pathway

In vertebrates, thyroid hormones are essential for post-embryonic development, such as establishing the central nervous system in mammals and metamorphosis in amphibians. The present paper summarizes the possible extra-thyroidal processes that environmental chemicals are known to or suspected to target in the thyroid hormone-signaling pathway. We describe how such chemicals interfere with thyroid-hormone-binding protein functions in plasma, thyroid-hormone-uptake system, thyroid-hormone-metabolizing enzymes, and activation or suppression of thyroid-hormone-responsive genes through thyroid-hormone receptors in mammals and amphibian tadpoles. Several organohalogens affect different aspects of the extra-thyroidal thyroid-hormone-signaling pathway but hardly affect thyroid hormone binding to receptors. Rodents and amphibian tadpoles are most sensitive to the effects of environmental chemicals during specific thyroid-hormone-related developmental windows. Possible mechanisms by which environmental chemicals exert multipotent activities beyond one hormone-signaling pathway are discussed.

Transthyretin RNA profiling in livers from transplanted patients affected by familial amyloidotic polyneuropathy, and identification of a dual transcription start point

Mutations in the transthyretin (TTR) gene cause familial amyloidotic polyneuropathy (FAP), an autosomal dominant peripheral neuropathy, often associated with cardiomyopathy. Liver transplant currently represents a powerful therapeutic approach for FAP patients, although its efficacy is heavily dependent both on the disease severity and on the cardiac functionality. We have investigated the TTR gene expression searching for tissue-specific additional messengers in human adult and foetal tissues as well as in eight livers from FAP transplanted patients carrying different TTR mutations (Met30, Pro36, Ala47, Arg50, and Gln89). We identified a novel transcript, recognising a different transcription start site. The additional 5'-UTR sequence of this novel transcript contains regulatory boxes possibly highlighting an additional transcription start point. RNA analysis revealed that this region is represented in all foetal/adult tissues analysed. We discussed the implications of this finding which might provide perspectives for better understanding the TTR gene expression.

Transthyretin constitutes a functional component in pancreatic beta-cell stimulus-secretion coupling

Transthyretin (TTR) is a transport protein for thyroxine and, in association with retinol-binding protein, for retinol, mainly existing as a tetramer in vivo. We now demonstrate that TTR tetramer has a positive role in pancreatic beta-cell stimulus-secretion coupling. TTR promoted glucose-induced increases in cytoplasmic free Ca(2+) concentration ([Ca(2+)](i)) and insulin release. This resulted from a direct effect on glucose-induced electrical activity and voltage-gated Ca(2+) channels. TTR also protected against beta-cell apoptosis. The concentration of TTR tetramer was decreased, whereas that of a monomeric form was increased in sera from patients with type 1 diabetes. The monomer was without effect on glucose-induced insulin release and apoptosis. Thus, TTR tetramer constitutes a component in normal beta-cell function. Conversion of TTR tetramer to monomer may be involved in the development of beta-cell failure/destruction in type 1 diabetes.

Binding of Amyloidogenic Transthyretin to the Plasma Membrane Alters Membrane Fluidity and Induces Neurotoxicity

Transthyretin (TTR) can deposit as amyloid in the peripheral nervous system and induce a peripheral neuropathy. We examined the mechanism of TTR amyloid neurotoxicity on SH-SY5Y neuroblastoma cells. Wild-type (WT) TTR and two amyloidogenic mutants (V30M and L55P) were expressed in Escherichia coli. Incubation (aging) of WT TTR at 37 degrees C for 1 week caused no significant aggregation. However, there was a significant increase in the extent of amyloid fibril formation after the amyloidogenic mutants had been aged. L55P TTR aggregated more readily than V30M TTR. Both amyloidogenic mutants were neurotoxic after aging. The order of neurotoxicity was as follows: L55P > V30M > WT. As binding of amyloid proteins to the plasma membrane may cause cytotoxicity, we studied the binding of TTR to a plasma membrane-enriched preparation from SH-SY5Y cells by surface plasmon resonance. All three forms bound to the plasma membrane through electrostatic interactions. The binding of the amyloidogenic mutants was increased by aging. The amount of binding correlated closely with the amount of aggregation and with the cytotoxicity of each form. As membrane fluidity can influence cell viability, we also examined the effect of TTR on membrane fluidity using a fluorescence anisotropy method. Binding of the amyloidogenic TTR mutants increased membrane fluidity, and once again, the order of potency was as follows: L55P > V30M > WT. These results demonstrate that TTR can bind to the plasma membrane and cause a change in membrane fluidity. Altered membrane fluidity may be the cause of the neurotoxicity.

Enrichment of low-abundant serum proteins by albumin/immunoglobulin G immunoaffinity depletion under partly denaturing conditions

We present a simple protocol for affinity depletion to remove the two most abundant serum proteins, albumin and immunoglobulin G (IgG). Under native conditions, albumin/IgG were efficiently removed and several proteins were enriched as shown by two-dimensional electrophoresis (2-DE). Besides that, partly denaturing conditions were established by adding 5 or 20% acetonitrile (ACN) in order to disrupt the binding of low-molecular-weight (LMW) proteins to the carrier proteins albumin/IgG. 2-DE results showed that the total number of detected LMW proteins increased under denaturing conditions when compared to native conditions. Interestingly, the presence of 5% ACN in serum revealed better enrichment of LMW proteins when compared to 20% ACN condition. Seven randomly distributed spots in albumin/IgG depleted serum samples under 5% ACN condition were picked from the 2-DE gels and identified by mass spectrometry (MS). The intensity of five LMW protein spots increased under denaturing conditions when compared to native conditions. Three of the seven identified spots (serum amyloid P, vitamin D-binding protein, and transthyretin) belong to a group of relatively low-abundant proteins, which make up only 1% of all serum proteins. The method presented here improves the resolution of the serum proteome by increasing the number of visualized spots on 2-D gels and allowing the detection and MS identification of LMW proteins and proteins of lower abundance.

Genetic and environmental contributions to serum retinol and alpha-tocopherol concentrations: the Stanislas Family Study

BACKGROUND

Although numerous environmental factors are documented to influence serum retinol and alpha-tocopherol concentrations, little is known about the genetic versus the environmental contributions to variations in these traits.

OBJECTIVE

The aim of this study was to estimate additive genetic heritability and household effects for serum retinol and alpha-tocopherol concentrations in a variance component analysis.

DESIGN

In a sample of 387 French families, information on serum retinol and alpha-tocopherol concentrations, usual dietary intake, lifestyle, and serum lipid profiles and related polymorphisms (apolipoprotein E, apolipoprotein C-III, apolipoprotein B, cholesteryl ester transfer protein, and lipoprotein lipase) was obtained.

RESULTS

For serum retinol--after adjustment for sex, age, body mass index, alcohol consumption, oral contraceptive use, and serum albumin, triacylglycerol, and apolipoprotein A-I concentrations--additive genetic effects and shared common environment contributed 30.5% and 14.2% of the total variance, respectively. For serum alpha-tocopherol, approximately 22.1% of the total variance was due to the additive effects of genes and 18.7% to those of household environment, after adjustment for the covariates sex, age, vitamin E intake, oral contraceptive use, and cholesterol, triacylglycerol, and apolipoprotein A-I concentrations. For both vitamins, the influence of measured polymorphisms was not significant. Moreover, heritability and household effect estimates were not significantly different between the 4 classes of relatives and did not vary significantly when families shared more meals at home.

CONCLUSIONS

The results show that serum retinol and alpha-tocopherol concentrations are under genetic control in healthy families.

Synthesis of transthyretin by the ependymal cells of the subcommissural organ

Transthyretin (TTR) is a protein involved in the transport of thyroid hormones in blood and cerebrospinal fluid (CSF). The only known source of brain-produced TTR is the choroid plexus. In the present investigation, we have identified the subcommissural organ (SCO) as a new source of brain TTR. The SCO is an ependymal gland that secretes glycoproteins into the CSF, where they aggregate to form Reissner's fibre (RF). Evidence exists that the SCO also secretes proteins that remain soluble in the CSF. To investigate the CSF-soluble compounds secreted by the SCO further, antibodies were raised against polypeptides partially purified from fetal bovine CSF. One of these antibodies (against a 14-kDa compound) reacted with secretory granules in cells of fetal and adult bovine SCO, organ-cultured bovine SCO and the choroid plexus of several mammalian species but not with RF. Western blot analyses with this antibody revealed two polypeptides of 14 kDa and 40 kDa in the bovine SCO, in the conditioned medium of SCO explants, and in fetal and adult bovine CSF. Since the monomeric and tetrameric forms of TTR migrate as bands of 14 kDa and 40 kDa by SDS-polyacrylamide gel electrophoresis, a commercial preparation of human TTR was run, with both bands being reactive with this antibody. Bovine SCO was also shown to synthesise mRNA encoding TTR under in vivo and in vitro conditions. We conclude that the SCO synthesises TTR and secretes it into the CSF. Colocalisation studies demonstrated that the SCO possessed two populations of secretory cells, one secreting both RF glycoproteins and TTR and the other secreting only the former. TTR was also detected in the SCO of bovine embryos suggesting that this ependymal gland is an important source of TTR during brain development.

Transthyretin, a new cryptic protease

Transthyretin (TTR) is a plasma homotetrameric protein that acts physiologically as a transporter of thyroxine (T(4)) and retinol, in the latter case through binding to retinol-binding protein (RBP). A fraction of plasma TTR is carried in high density lipoproteins by binding to apolipoprotein AI (apoA-I). We further investigated the nature of the TTR-apoA-I interaction and found that TTR from different sources (recombinant and plasmatic) is able to process proteolytically apoA-I, cleaving its C terminus after Phe-225. TTR-mediated proteolysis was inhibited by serine protease inhibitors (phenylmethanesulfonyl fluoride, Pefabloc, diisopropyl fluorophosphate, chymostatin, and N(alpha)-p-tosyl-l-phenylala-nine-chloromethyl ketone), suggesting a chymotrypsin-like activity. A fluorogenic substrate corresponding to an apoA-I fragment encompassing amino acid residues 223-228 (Abz-ESFKVS-EDDnp) was used to characterize the catalytic activity of TTR, including optimum reaction conditions (37 degrees C and pH 6.8) and catalytic constant (K(m) = 29 microm); when complexed with RBP, TTR activity was lost, whereas when complexed with T(4), only a slight decrease was observed. Cell lines expressing TTR were able to degrade Abz-ESFKVS-EDDnp 2-fold more efficiently than control cells lacking TTR expression; this effect was reversed by the presence of RBP in cell culture media, therefore proving a TTR-specific proteolytic activity. TTR can act as a novel plasma cryptic protease and might have a new, potentially important role under physiological and/or pathological conditions.

Identification of S-sulfonation and S-thiolation of a novel transthyretin Phe33Cys variant from a patient diagnosed with familial transthyretin amyloidosis

Familial transthyretin amyloidosis (ATTR) is an autosomal dominant disorder associated with a variant form of the plasma carrier protein transthyretin (TTR). Amyloid fibrils consisting of variant TTR, wild-type TTR, and TTR fragments deposit in tissues and organs. The diagnosis of ATTR relies on the identification of pathologic TTR variants in plasma of symptomatic individuals who have biopsy proven amyloid disease. Previously, we have developed a mass spectrometry-based approach, in combination with direct DNA sequence analysis, to fully identify TTR variants. Our methodology uses immunoprecipitation to isolate TTR from serum, and electrospray ionization and matrix-assisted laser desorption/ionization mass spectrometry (MS) peptide mapping to identify TTR variants and posttranslational modifications. Unambiguous identification of the amino acid substitution is performed using tandem MS (MS/MS) analysis and confirmed by direct DNA sequence analysis. The MS and MS/MS analyses also yield information about posttranslational modifications. Using this approach, we have recently identified a novel pathologic TTR variant. This variant has an amino acid substitution (Phe --> Cys) at position 33. In addition, like the Cys10 present in the wild type and in this variant, the Cys33 residue was both S-sulfonated and S-thiolated (conjugated to cysteine, cysteinylglycine, and glutathione). These adducts may play a role in the TTR fibrillogenesis.

Elevated plasma factor VIII in a mouse model of low-density lipoprotein receptor-related protein deficiency

It has been established that low-density lipoprotein receptor-related protein (LRP) is involved in the cellular uptake and degradation of coagulation factor VIII (FVIII) in vitro. To address the physiologic role of LRP in regulating plasma FVIII in vivo, we used cre/loxP-mediated conditional LRP- deficient mice (MX1cre(+)LRP(flox/flox)). Upon inactivation of the LRP gene, MX1cre(+)LRP(flox/flox) mice had significantly higher plasma FVIII as compared with control LRP(flox/flox) mice (3.4 and 2.0 U/mL, respectively; P <.001). Elevated plasma FVIII levels in MX1cre(+)LRP(flox/flox) mice coincided with increased plasma von Willebrand factor (VWF) (2.0 and 1.6 U/mL for MX1cre(+)LRP(flox/flox) and control LRP(flox/flox) mice, respectively; P <.05). Elevation of plasma FVIII and VWF persisted for at least 6 weeks after inactivation of the LRP gene. Upon comparing plasma FVIII and VWF in individual mice, we observed an increase of the FVIII/VWF ratio in MX1cre(+)LRP(flox/flox) mice as compared with control LRP(flox/flox) mice. Administration of either a vasopressin analog or an endotoxin resulted in increased plasma VWF, but not FVIII. In clearance experiments, MX1cre(+)LRP(flox/flox) mice displayed a 1.5-fold prolongation of FVIII mean residence time. Adenovirus-mediated overexpression of the 39-kDa receptor-associated protein (RAP) in normal mice resulted in a 3.5-fold increase of plasma FVIII. These data confirm that the regulation of plasma FVIII in vivo involves a RAP-sensitive mechanism. Surprisingly, plasma FVIII in MX1cre(+)LRP(flox/flox) mice increased 2-fold after RAP gene transfer. We propose that RAP-sensitive determinants other than hepatic LRP contribute to the regulation of plasma FVIII in vivo.